Childhood body fatness (BF) is inversely associated with breast cancer (BC) risk. As BC occurs much later, breast density is used as a surrogate marker. We evaluated whether BF trajectories during childhood and adolescence are associated with breast composition after puberty and before first pregnancy.

The Growth and Obesity Cohort Study followed 602 Chilean girls from age 3.5 with annual BF assessments: body mass index, waist circumference, fat mass percentage, and fat mass index. Breast composition—% fibroglandular volume (%FGV) and absolute fibroglandular volume (AFGV)—was measured using DXA at 4 years post-menarche (4 PM) and at age 19. BF trajectories were derived via Growth Mixture Models. Associations with breast composition were assessed using linear regression models.

Data from 322 girls at age 19 showed 21.2% obesity and mean %FGV of 46.3%. All BF markers and trajectories during childhood/adolescence were inversely associated with %FGV at 19 y. We did not observe associations with BF and change in %FGV and AFGV between 4 PM and 19 y. Adiposity markers at 4 PM and 19 y were positively associated with AFGV, but not BF trajectories. Girls who gained more BMI and WC between 4 PM and 19 y had higher AFGV at age 19.

We observed an inverse association between BF during childhood/adolescence and %FGV, supporting previous evidence. However, concurrent BF was positively associated with AFGV, contrasting previous findings. Further research is needed to understand fat distribution patterns during these critical periods and their implications for breast tissue composition.